DIY Doctor UK DIY Forums

Any help on the following very much appreciated. I've spent thousands on a new system a year ago and the original plumber no longer keen to help, so hopefully someone out there can.

System: Worcester 28KW system boiler + flowmaster (unvented)
The original pipework has been added to several times but no problems visible. Old house. 3 floors. 20 rads.

Problem: some knocking in the pipes although all the pipes have been secured. the boiler itself is quiet. More annoyingly, two rads on the first floor and one on the second always need bleeding. Pressure does not seem to fall on the boiler but it's a large system with a small extra expansion tank, so perhaps not large enough drop to be obvious.

The tall towel rail in the bathroom is main culprit with sometimes 10 seconds worth of air. There is no sign of water leakage anywhere.

I would like to know: Can air enter the system without pushing out water, if so, how and where?

I suspect a couple of TRVs are sited on the return instead of flow - does that matter if they are reversible?

Do I need a bypass valve? TRVs on every rad apart from bathroom which has, it seems, two lockshields. I have one TRV on 4/5 - higher than the rest to act as a bypass at the moment.

If i fit a valve to automatically bleed air, does that really help, since air is coming in from somewhere?

And can the vent pipe leak air into the system?

If anyone can help, would be great. Thanks Dan

Some trvs which are reversable have to be set inside the head for flow direction

Three floors of heating require a extra expansion vessel as the boilers inbuilt one is only sized to suit two floors. Yes you need a by-pass valve and this has been part of the building regs since April 2006. Yes it matters if the TRV's have been fitted on the return even if they are bi-directional as they are installed differently if fitted on the return. Each of these points are reason enough to challenge your installer to fix his cock-ups or take him to trading standards, report him to your local building control and even to CORGI ref the expansion vessel.

Thanks for these replies, very useful.

Could you clarify how to fit 'against the head' if you are refitting a trv that's reversible? I'd like to check if they have done it correctly. Am I right that if heating is on, hot pipe into rad is flow, cold is return. If both are hot, this is incorrect?

Also, can you tell me air can enter a pressurised system without expelling water? Can air enter the TRV if it's incorrectly fitted?

Is there any point fitting a valve that bleeds air out? (presumably the pressure in the booielr will simply keep falling?)

And where do you fit the by-pass valve?

I'd like to send someone a bottle of wine - thanks for your help

Dan